Enhanced Photocatalytic Hydrogen Evolution by Integrating Dual

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Enhanced Photocatalytic Hydrogen Evolution by Integrating Dual Cocatalysts on Heterophase CdS Nano-Junctions D. Amaranatha Reddy, Eun Hwa Kim, Madhusudana Gopannagari, Rory Ma, Palagiri Bhavani, D. Praveen Kumar, and Tae Kyu Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02098 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Sustainable Chemistry & Engineering

Enhanced Photocatalytic Hydrogen Evolution by Integrating Dual Co-Catalysts on Heterophase CdS Nano-Junctions D. Amaranatha Reddy, Eun Hwa Kim, Madhusudana Gopannagari, Rory Ma, P. Bhavani, D. Praveen Kumar and Tae Kyu Kim* Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, Busan 46241, South Korea *E-mail address: [email protected]

KEYWORDS: CdS nano-junctions, sunlight-driven photocatalyst, H2 production, co-catalysts, high stability.

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ABSTRACT Development of novel low price porous nanostructures with robust photocatalytic hydrogen generation rate and high durability is critical to help to meet the future energy demand. A prominent number of sunlight active semiconductor photocatalyst nanostructures have been formulated for the aforementioned photocatalytic reactions. However, their practical application has been limited by low efficiency and un-stability induced by the rapid recombination of charge carriers. To effectively reduce the recombination rate, the addition of electron-transporting cocatalysts is a promising strategy. However, the rate of photogenerated holes is generally slower than that of photogenerated electrons, extending the recombination. To overcome this difficulty in this study for the first time, co-loading of both photogenerated electrons and hole-transporting co-catalysts (C@CoS2 and TFA) on light-harvesting semiconductor heterophase homojunction CdS (OD-2D CdS) is established as a productive way to ameliorate the photocatalytic water splitting efficiency. Benefiting from the huge active catalytic sites, high light harvesting capacity and suitable band structure, the nanohybrid exhibits a prominent amount of hydrogen 87.73 mmol·gcat-1·h-1 was evolved with high durability. We believe that the results presented herein may expand the potential uses of sunlight active catalysts for sustainable and clean H2 fuel production and to help satisfy the future energy demand.


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INTRODUCTION The production of clean H2 fuel by photocatalytic water splitting using novel semiconductor nanostructures is an ideal strategy to replace the current fossil fuels.1,2 Many semiconductor photocatalyst nanostructures have been developed for the aforementioned catalytic reaction.3-6 Of these semiconductor nanostructures, cadmium sulfide (CdS) nanostructures have been shown to be one of the most active photocatalysts for efficient H2 production because of its favorable band structure.7 However, its practical application is limited by its poor photo-stability.8 To overcome these drawbacks, strategies like doping, adding co-catalysts, forming composites with suitable nanostructures or conductive carbon-based materials, and phase junction formation using semiconductor polymorphism have been recently developed.9-11 Advances in highly ordered heterophase homo-junction formation offer exciting possibilities to direct the electron flow on the phase interface and effectively separate photocharge carriers to hinder their recombination rate and promote H2 evolution.12-15 For instance, Due et al.12 fabricated elongated nanocrystals with highly crystalline heterophase (zinc blende/wurtzite) homo-junction (Cd1−xZnxS) using solvothermal route and noticed that the designed nanostructures exhibit extraordinary visible light photocatalytic hydrogen due to efficient charge separation by the internal electrostatic field resulting from the band formation of zinc blende/wurtzite junctions. Ai et al. designed novel CdS phase junction nanostructures with tunable phase junctions and reduce the recombination rate during the catalytic reaction.13 Li et al.14 designed CdS nano-phase junctions composed of hexagonal cores and cubic shells by directly reacting Cd(NO3)2 and thiourea precursors for photocatalytic hydrogen production. Shen et al.15 developed the heterophase homo-junction Zn0.5Cd0.5S nanostructures and noticed robust hydrogen evolution rate under visible light irradiation. 3 ACS Paragon Plus Environment


Although the formation of heterophase homo-junctions within the single dimension of nanostructures has been established as a promising strategy, there are various obstacles, which can limit high hydrogen evolution rates: (i) During the catalytic reaction, the phase junction can be collapsed due to oxidized of photogenerated holes, thus reducing the hydrogen evolution rate. (ii) For some cases, after forming the heterophase homo-junctions, oxidation and reduction potentials of nanocomposites are insufficient for photocatalytic reactions.16 To overcome these difficulties, recently a new strategy for making heterophase homo-junctions by coupled interfaces of two different dimensional nanostructures (0D−1D, 0D-2D, 1D−1D, 2D−2D, and 1D−2D) are developed. There strategies can tune the oxidation or reduction potentials by changing the size or dimension of the nanostructures.17 Particularly, the formation of 0D-2D nanostructures has attracted much attention due their peculiar advantages such as intimate interface contact between 0D and 2D favors the exciton dissociation, resulting enhancing the hydrogen evolution rate. Moreover, the large lateral size of 2D with high surface area of 0D nanostructures leads to large contact area, which makes prominent photo charge carriers transfer and migration, thereby significantly enhanced hydrogen evolution rates.18 However, in the literature cited above, the separation of photo-charge carriers and migration mainly occur at the phase junction interface, rather than on the outer surface of the photocatalyst. As is well-known, the final photocatalytic H2 reactions mainly take place on the surface of the photocatalyst.19 To overcome this challenge, Silva and co-workers inserted Pt nanocrystals as a co-catalyst on the surface of phase junctions and doubled the photocatalytic hydrogen evolution rate,20 but these inserted Pt nanocrystals limit the commercialization of CdS hybrid nanostructures; due to their high price and scarcity. Hence, the development of a novel


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low price noble metal free electron transporting co-catalysts with robust efficiency is a significant challenge.21 Recently, numerous noble-metal-free electron transporting co-catalysts have been developed for photocatalytic H2 generation.21 Among the studied co-catalysts, cobalt sulfide based nanostructures have much attention due to their suitable band edge reduction potentials and narrow band gap.21 However, these nanostructures suffer from limited conductivity and porosity which leads to low surface catalytically active sites and low photo charge-carriers separation efficiency resulting decrease the H2 evolution rate. To overcome this difficulty in the present study we have designed carbon layers wrapped CoS2 hollow nanostructures through MOF template engaged strategy. We expect that the wrapping of thin carbon layers on the CoS2 nanostructures alter the electronic structure and contribute it for improved charge carrier’s separation. Furthermore, it can protect the nanocrystals from photocorrosion and aggregation during the catalytic reaction. In addition to electron transportation, the hole shuttling is also significantly important for efficient hydrogen production. Generally in photocatalytic reaction the transport rate of the photogenerated holes is slower than the photogenerated electrons, which causes the high photoelectron-hole recombination rate. To overcome this difficulty recently several hole transporting agents such as RuO2, IrO2 and water soluble TFA solution are utilized and improve the catalytic efficiency.22 Interestingly up to now, most of the researchers utilized either electron transporting agents or hole transporting agents individually as a co-catalyst for enhancing the efficiency of photocatalytic H2 evolution. Inorder to further enhance the H2 evolution rate, we believe that both electron and hole transporting co-catalysts loading is a novel and new strategy to attain high-efficiency photo-charge carrier separation.21,22 Unfortunately, this strategy of co5 ACS Paragon Plus Environment


loading both electron- and hole-transporting co-catalysts has rarely been used for photocatalytic hydrogen evolution reactions, especially for CdS phase junction-based photocatalysts. Considering the importance of co-loading electron- and hole-transporting co-catalysts and formation of CdS phase junctions, we designed a novel highly robust photocatalytic nanohybrid for solar-driven H2 production by water splitting. The designed photocatalytic system contains heterophase nano-junctions of 0D CdS mesoporous networks on porous single-crystal 2D CdS nanosheets (0D-2D CdS) as a sunlight-harvesting material. In addition to heterophase CdS nanojunctions, few-layered carbon-anchored CoS2 nanoparticles and water-soluble trifluoro acetic acid (TFA) were used as co-catalysts to reduce to the recombination rate of CdS nanostructures. Benefiting from an abundance exposed edge active sites and rich vacancy states, the nanohybrid exhibits a rapid H2 production about 87.73 mmol·gcat-1·h-1 under solar light. We conceive that the demonstrated design strategy for the sustainable H2 production will help to meet the future global energy demand.

RESULTS AND DISCUSSION The multistep synthetic process to obtain 0D-2D CdS@C-CoS2 nanostructures is depicted in Scheme 1. In step I, the 2D CdS nanosheets are first synthesized by a cation-exchange reaction between ZnS-DETA (diethylenetriamine) and Cd2+ using the hydrothermal method.23 Then, the 0D CdS mesoporous nanostructures are deposited on the 2D CdS using a polymer-templated aggregation strategy to form the 0D-2D CdS phase junction. In step II, the C@CoS2 nanostructures were synthesized by sulfidation of Co@C nanostructures with sulfur at 300 °C/2h. Finally, for the preparation of CdS/C@CoS2 nanocomposites, the 0D-2D phase junction


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CdS nanostructures and C@CoS2 nanostructures were mixed in a homogeneous chemical solution process. The detailed experimental synthesis process of different CdS (0D, 1D, 2D, 0D1D, 0D-2D and 2D-1D) nanostructures, CdS/C@CoS2, and C@CoS2 nanocomposites are presented in the Supporting Information (SI) experimental section.

Scheme 1. Schematic illustration of the preparation of C@CoS2 nanostructures and 0D-2D CdS/C@CoS2 nanohybrids The surface microstructure and morphology of the as-synthesized nanostructures were examined using FESEM and FETEM as shown in Figure 1. Figure 1(a) shows an FETEM micrograph of the 0D-CdS nanocrystals prepared through polymer template aggregation. The micrograph clearly demonstrates that an irregular assembled mesoporous networks consisting of connected CdS nanoparticles. Further, the examination of the HRTEM micrograph reveals the average size of the CdS nanoparticles is around 2 nm (Figure S1(a)) and distinguishable set of lattice diffraction planes with an inter-planar distance of 0.34 nm, which well matches the dspacing of the (111) plane of mesoporous CdS nano-networks (Figure S1(b)).24 Figure 1(b) demonstrates the FESEM diagram of the 1D-CdS nanostructures, which confirms the 7 ACS Paragon Plus Environment


synthesized nanostructures are rod like morphology and their average size is approximately 15– 20 nm in width and 110–200 nm in length. Figure 1(c) shows the as-prepared 2D CdS nanostructures with ultra-long nanosheet-like morphology formed by a mass of radially arranged quantum dots. The mean length of the nanosheets is about 500–600 nm and the width is approximately 100–150 nm. To confirm the mesoporous nature of the synthesized nanostructures (0D, 1D and 2D-CdS), N2 adsorption–desorption isotherms were measured as shown in Figure S2. The nanostructures exhibit a hysteresis loop which is nearly a type IV isotherm, confirming the mesoporous nature. In addition, the pore diameter and pore volumes were estimated using BJH method (Figure S2). The estimated average pore diameters and pore volumes are 0D-CdS (16.96 nm and 0.902 cm3/g), 1D-CdS (8.26 nm and 0.010 cm3/g) and 2D-CdS (12.49 nm and 0.022 cm3/g), respectively. The FESEM image of the 1D-0D CdS nanostructures demonstrates that 0D mesoporous nanostructures are anchored on the outer area of the 1D-CdS nanorods and formed the 1D-0D CdS hetero-structure (Figure 1(d)). The FESEM images of the 1D-2D CdS nanostructures are presented in Figure 1(e). In contrast to the large 1D CdS nanocrystals, the 2D-CdS nanosheets are attached and form a hetero-structure. Figure 1(f) shows the FESEM image of the 0D-2D nanostructures, which is indicative of uniform mesoporous CdS nanocrystals formed on the surface of the 2D nanosheets. The mean length of the 2D nanosheets is about 500–600 nm and the width is approximately 100–150 nm. The measured size of the 0D CdS nanostructures on the surface of the 2D nanostructures is 2-3 nm. FETEM and HRTEM images (Figure 1(g-i)) reveal that cubic@hexagonal heterophase junctions were formed within the nanostructures. Figure 1(j) shows an FETEM image of the as-prepared ZIF-67 polyhedral morphology with mean size of 200 nm.24 Figure S1(C) shows the FESEM image of the C@CoS2 nanostructures derived from the sulfidation reaction with Co@C and sulfur powder at


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300 °C for 2 h under a nitrogen atmosphere, which reveals that the rhombic dodecahedrons were totally converted to a C@CoS2 nanostructure. More interestingly, the C@CoS2 nanostructures are still maintained polyhedral morphology, indicating their robust structural stability. However, the graphitic carbon layers were not clearly noticed in the FESEM image due to their small size and thin nature. A closer examination using FETEM and HRTEM analysis on the C@CoS2 nanostructures reveals that the nanostructures consist of CoS2 nanocrystals anchored with layered graphitic carbon shell (Figures 1(k) and 1(l)). The HRTEM image in Figure S1(d) clearly shows that the adjacent lattice diffraction planes have a d-spacing of 0.27 nm, corresponding to (200) planes of CoS2. A representative FETEM image of the 0D-2D CdS/C@CoS2 nanocomposite (Figure 2(a)) reveals that the porous C@CoS2 nanostructures are anchored to the surface of the 0D-2D CdS nanostructures. This was further confirmed by the elemental mapping analysis results showing a similar elemental composition of CdS/C@CoS2 (Figure 2(d-i)). The HRTEM images (Figure 2(b, c)) also confirm that the nanocomposite consists of heterophase CdS nano-junctions and C@CoS2 nanostructures.



Figure 1. FETEM images of (a) 0D CdS nanostructures prepared through polymer template aggregation. (b–f) FESEM images of 1D CdS, 2D CdS, 1D-0D CdS, 1D-2D CdS, and 2D-0D nanostructures, respectively. (g) FTEM image of 0D-2D nanostructures. (h-i) HRTEM images of 0D-2D CdS nanostructures. (j) FETEM image of ZIF-76 nanostructures. (k) FETEM image of C@CoS2 nanostructure and (l) the corresponding HRTEM image.


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Figure 2. (a) FETEM image of the 0D-2D CdS/C@CoS2 nanohybrid. (b and c) Corresponding HRTEM images of the 0D-2D CdS/C@CoS2 nanohybrid. (d) HAADF image and (e–i) elemental mapping of the CdS/C@CoS2 nanohybrid, showing the distribution of (e) Cd, (f) S, (g) Co, (h) C, and (i) all elements in the CdS/C@CoS2 nanohybrid.

The crystallographic structure, phase, and purity of the CdS and CdS/C@CoS2 were examined by XRD (Figure 3(a)). The broad peaks of low intensity arising from the 0D-CdS nanostructures suggest a very small size of the synthesized mesoporous nanostructures. The 2DCdS XRD patterns confirmed that the samples were typical hexagonal wurtzite CdS (JCPDS 4111 ACS Paragon Plus Environment


1049). The high intensity and sharp diffraction peaks indicate that the 2D-CdS nanostructures are extremely crystalline. For the 0D-1D CdS and 2D-0D CdS nanostructures, the XRD patterns revealed two distinct sets of lattice diffraction peaks related to cubic and hexagonal, respectively. The XRD patterns of the 0D-2D CdS/C@CoS2 nanostructures are match to that of 0D-2D CdS nanostructures. Interestingly, the introduction of the C@CoS2 nanostructures does not affect the overall 0D-2D CdS structure since no additional XRD peaks appear upon formation of the nanocomposite. Relatively small amount and low diffraction intensity of C@CoS2 is the reason. The XRD diffraction pattern of the ZIF-67 matches with the XRD diffraction planes simulated using the standard ZIF-67 (Figure S3). The XRD diffraction patterns (Figure S3) of Co@C suggests the complete transformation from ZIF-67 to Co@C. The C@CoS2 nanostructures could be clearly indexed to the pure cubic phase of CoS2, and they match well with the standard data (JCPDS card no. 65-3322). Further, XPS analysis was performed to examine the chemical state and purity of the optimized 0D-2D CdS/C@CoS2 nanostructures. As depicted in Figure 3(b), the XPS survey spectrum indicates the presence of Cd, S, C, and Co in the CdS/C@CoS2 nanohybrid. In the core level XPS spectra, Cd and S are split into a doublet at 405.24, 411.98, 161.14, and 162.35 eV, which match to Cd 3d5/2, Cd 3d3/2, S2p3/2, and S 2p1/2, respectively (Figure 3(c, d)).25,26 Cd and S binding energies demonstrate both in their +2 oxidation states. The C 1s narrow-scan spectrum (Figure 3(e)) of the CdS/C@CoS2 nanocomposite could be deconvoluted into three peaks located at binding energies of 284.20, 285.17, and 288.05 eV. The sharp and high intensity peak at 284.20 eV can be assigned to sp3 C of the CdS/C@CoS2; Other two low-intensity peaks located at around 285.17, and 288.05 eV, are due to the C−O and C=O type carbons, respectively.27 The core level XPS Co 2p spectrum (Figure 3(f)) shows the typical spin-orbit


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coupled Co 2p3/2 and Co 2p1/2 peaks at 781.44 and 796.61 eV, respectively. The binding energy difference between Co 2p3/2 and 2p1/2 is approximately 15 eV, which indicates the cobalt ion in the CoS2 compound is in +2 oxidation state. The satellite peaks located at approximately 786.61 and 802.66 eV can be assigned to the shakeup excitation of the high-spin Co2+ ions.28 The XPS results strongly confirm the successful fabrication of the CdS/C@CoS2 nanocomposite.

Figure 3. (a) XRD patterns of 0D CdS, 2D CdS, 1D-0D CdS, 0D-2D CdS, 1D-2D CdS and 0D2D CdS/C@CoS2 nanohybrid. (b) XPS full survey spectrum of the 0D-2D CdS/C@CoS2 nanohybrid. ((c)–(f)) Narrow-scan spectra of Cd 3d, S 2p, C 1s and Co 2p species in the 0D-2D CdS/C@CoS2 nanocomposite, respectively.

The UV-vis DRS of the as-prepared 0D-CdS, 1D-CdS, 2D-CdS, 0D-1D CdS, 1D-2D CdS, 0D-2D CdS, and 0D-2D CdS/C@CoS2 nanostructures are shown in Figure 4(a). The asprepared 0D-CdS mesoporous nanostructures show a sharp absorption edge at around 472 nm,



which is related to the intra-band transitions of the CdS nanostructures.29 From this value, the band gap of the 0D-CdS mesoporous nanostructures is calculated to be 2.62 eV using the Kubelka–Munk relation. The measured band gap value is much larger than that of the bulk CdS (2.4 eV), which can be ascribed to their relatively small size (2 nm) and quantum-size effects. Compared to the 0D-CdS nanostructures, the absorption edges of 1D-CdS (514 nm) and 2D-CdS (511 nm) shift towards the higher wavelengths, thus indicating decreased band gap values. The measured band gap values of 1D-CdS and 2D-CdS are calculated to be 2.42 and 2.41 eV, respectively, which are considerably lower than that of 0D-CdS nanostructures. Variations of the steep absorption edge and related band gap energies can be possibly due to their different nanocrystallite sizes,30 however, after forming heterophase 0D-1D and 0D-2D CdS, the absorption edge maxima are blue-shifted, indicating that the 0D CdS mesoporous nanostructures are successfully deposited on 1D and 2D CdS and are used for band gap tuning. Finally, the 0D-2D CdS nanocomposite with C@CoS2 nanostructures shows enhanced absorption intensity in the range of 300–800 nm, which indicates 0D-2D CdS/C@CoS2 nanostructure have a higher light harvesting capacity than that of 0D-2D CdS, leading to enhanced catalytic efficiency.31 Figure 4(b) illustrates the photoluminescence (PL) spectra for the 0D-CdS, 1D-CdS, 2DCdS, 0D-2D CdS, and 0D-2D CdS nanohybrids, which show the PL maxima at 511, 529, 530, 503 and 501 nm, respectively. The broad PL peaks could be assigned to the near band edge emissions.32 0D-CdS, 1D-CdS and 2D-CdS nanostructures shows high intensity emission band, which is indicative of the strong recombination of photo-generated electron and hole pairs. However, upon the formation of heterophase homo-junctions such as 0D-2D CdS and 0D-1D CdS nanostructures, an appreciable emission intensity drop and a slight blue-shift in PL maximum are noticed. Especially, 0D-2D CdS nanostructures, showing very low intensity drop


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than the other nanostructures suggest the photo charge recombination is greatly suppressed by making improved photo charge carriers separation. The efficient charge recombination may be owed to the formation of heterophase and its suitable band structure. Efficient charge transfers could be occurred when cubic phase CdS is transformed to the hexagonal phase CdS. To understand the role of TFA and C@CoS2 nanostructures as a co-catalysts, we have measured the PL spectra of 0D-2D CdS, 0D-2D CdS + TFA, 0D-2D CdS/C@CoS2 and 0D-2D CdS/C@CoS2+TFA and the results are presented in Figure 4(c). It is evident that 0D-2D CdS/C@CoS2 + TFA nanostructures exhibited lower intensity emission than those of 0D-2D CdS, 0D-2D CdS + TFA and 0D-2D CdS/C@CoS2 nanostructures. The low-intensity emission spectra suggest that the photo charge recombination is greatly suppressed by making improved photo charge carriers separation.33 It can be deduced that the loaded TFA and C@CoS2 nanostructures have a stronger impact on the 0D-2D CdS in facilitating the effective separation and migration of photo-induced electron-hole pairs. The nanostructures also reduce the recombination rate, resulting in more photogenerated electrons than can be mobilized for the reduction of H+ to H2.34



Figure 4. (a) DRS spectra (b) PL spectra of 0D CdS, 1D CdS, 2D CdS, 1D-0D CdS, 0D-2D CdS, 1D-2D CdS, and 0D-2D CdS/C@CoS2 nanostructures. (c) PL spectra of 0D-2D CdS, 0D2D CdS after adding TFA solution, 0D-2D CdS/C@CoS2 and 0D-2D CdS/C@CoS2 + TFA solution. (d) Photocurrent analysis of C@CoS2, 0D-2D CdS and 0D-2D CdS/C@CoS2 nanostructures.

The separation of the photo charge carriers, migration efficiency, and photo-induced charge carrier transport of the 0D-2D CdS, C@CoS2 and 0D-2D CdS/C@CoS2 nanostructures were measured using photocurrent response of three consecutive on/off lighting cycles in 50 s intervals (Figure 4(d)). The C@CoS2 nanostructures show very low photo-current (0.25 × 10-7 A) due to its limited light harvesting capacity. The 0D-2D CdS nanostructures shows much higher photocurrent (0.65 × 10-7 A) compared to the C@CoS2 nanostructures, which may be due 16 ACS Paragon Plus Environment

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to the high visible light harvesting capacity of CdS nanostructures. Moreover, the photocurrent intensity quickly decreased when the light source turns it off, suggesting that the generated photo electrons migrate to the FTO electrodes to generate the photocurrents under light irradiation.35 The 0D-2D CdS/C@CoS2 composite nanostructures shows much higher photocurrent density (2.3 × 10-7 A) than those of 0D-2D CdS nanostructures and C@CoS2, which can be ascribed to the close interfacial contact and synergetic effect between the 0D-2D CdS and C@CoS2; C@CoS2 acts as a favorable co-catalyst and helps to separation of photogenerated charge carriers, thus greatly inhibits the direct recombination of photogenerated electrons and holes.3 Therefore, C@CoS2 nanostructures can significantly contribute to the robust H2 productions. In addition to the photocurrent measurements, photogenerated charge carrier separation was further confirmed by electrochemical impedance analysis. Figure S4 shows impedance analysis of the 0D-2D CdS and 0D-2D CdS/C@CoS2 nanohybrid. It is clear that 0D-2D CdS/C@CoS2 shows a smaller semi-circle than that of pure 0D-2D CdS. Normally, a smaller radius semi-circle indicates that the synthesized material has higher conductivity. From the results, it can be clear that as-synthesized 0D-2D CdS/C@CoS2 nanohybrid is superior at promoting the photogenerated charge-carriers transfer from 0D-2D CdS.36 Once the photogenerated charge-carriers are separated and migrated, the photocatalytic H2 production is affected by the surface catalytic redox reactions, which is facilitated with active sites on 0D-2D CdS/C@CoS2 nanohybrid. To verify the active sites, we measured the nitrogen adsorption–desorption (BET) curves. The BET results demonstrate that the surface area of the 0D-2D CdS/C@CoS2 nanohybrid (185.39 m2·g-1) is much higher than that of the 0D-2D CdS



nanorods (125.84 m2·g-1); the higher specific surface area may contribute to participate more number of catalytically active sites to boost the solar driven hydrogen evolution. The enhanced H2 evolution mainly depends on the production of the photo charge carriers and their shutting ways of path that strongly depend on the conduction band (CB) potentials of the semiconductor nanostructures. To determine the suitability of the CB potentials of the as-prepared 0D-CdS, 2D-CdS, 0D-2D CdS and C@CoS2 nanostructures, we executed Mott-Schottky (MS) analyses and compared the results to the standard reduction potentials of H+ to H2. The calculated flat band potentials of 0D-CdS, 2D-CdS, 0D-2D CdS and C@CoS2 determined by MS analyses are approximately –0.78, –0.76, –0.75 and –0.61 V vs NHE, respectively (Figure S5). These results demonstrate that the conduction band potential of 2D CdS is slightly positive than the 0D-CdS nanostructures. It indicates that the excited electrons from the CB of 0D-CdS can easily transfer to 2D CdS due to suitable band edge potential, resulting reduced recombination rate during the catalytic reaction. Moreover, we have noticed that after forming the 0D-2D CdS nanocomposite, the CB potential positively shifted compared to the individual 0D and 2D CdS nanostructures. The slight positive shift may be due to efficient close interfacial contact between 0D-CdS nanostructures and 2D-CdS nanostructures. When such close interfacial contact was made, the photogenerated electrons from the CB of 0D-CdS can flow to the CB of 2D-CdS which can lower their energy resulting potential drop arise across the junction of 0D-CdS and 2D-CdS. This potential drop may continuous up to 0D-CdS reaches the equilibrium with the Fermi level of 2D-CdS. Moreover, this potential drop may cause the deformation of the band structure. We expect that the deformation of band structure and positive shift may significantly contribute the efficient photocatalytic processes.31 Moreover, the cocatalyst C@CoS2 conduction band is more negative than the reduction potential of H+/H2O and


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more positive than the 0D-2D CdS nanostructures indicating that the CB electrons of the 0D-2D CdS nanostructures can easily shuttled to the CB of C@CoS2 and used for H2 production.37,38 The photocatalytic activities of the synthesized nanostructures are shown in Figure 5. First, the photocatalytic hydrogen evolution rate was optimized with different CdS nanostructures (0D-CdS, 1D-CdS, 2D-CdS, 0D-1D CdS, 1D-2D CdS, and 0D-2D CdS) in the presence of different scavengers combination under sunlight irradiation (Figures 5(a, b) and Figure S6). The results demonstrated that the 0D-2D CdS nanostructures prepared with a lactic acid (3 mL) and TFA (200 µL) mixture showed robust H2 production rate of approximately 14.86 mmol·gcat–1·h–1, which is much greater than the 0D-CdS, 1D-CdS, 2D-CdS, 0D-1D CdS, and 1D-2D CdS nanostructures. The improved photocatalytic hydrogen production rate of the synthesized 0D-2D CdS nanostructures may be due to several factors. First, the establishment of phase junctions between the cubic 0D CdS and 2D hexagonal CdS nanostructures serves as an energy transition area that minimizes energy loss, helps to transport charge carriers, and reduces the recombination rate owing to its favorable band structure.13 Moreover, photoluminescence analysis (Figure 4(b)) clearly shows that the recombination rate is associated with the formation of phase junctions. Second, the water-soluble molecular co-catalyst serves as an accelerating hole-transfer agent, which greatly reduces the photo recombination rate and improves the H2 evolution rate (Figure 4(c)).22 Since the 0D-2D phase junction nanostructures showed greatly improved photocatalytic H2 production after the addition of TFA, electron-transporting C@CoS2 nanostructures cocatalysts were integrated into the 0D-2D CdS to further improve the H2 production rate of the 0D-2D CdS nanostructures. This was achieved using the consecutive chemical adsorption process with ultrasonication, and the resulting nanostructures were assessed for H2 production 19 ACS Paragon Plus Environment


under sunlight irradiation as shown in Figure 5(c). Results demonstrated that the C@CoS2 nanostructures integrated on 0D-2D CdS produces a higher amount of H2, which increases with the loading amount of C@CoS2 nanostructures up to the optimal level of 8%; this optimized nanohybrid exhibits an enhanced amount of photocatalytic H2 around 87.73 mmol·gcat–1·h–1. The identical experimental condition was repeated up to five repeated time and noticed almost similar H2 evolution rate (Figure 5(d)). This rate is approximately six times larger compared to the bare 0D-2D heterophase CdS nanostructures H2 evolution. Furthermore, the observed hydrogen evolution rate is much higher than that of 0D-CdS/C@CoS2, 1D CdS/C@CoS2, 0D-1D CdS/C@CoS2, 1D-2D CdS/C@CoS2 and doped CdS/Pt nanostructures (Figures S7 and S8). Moreover, the observed rate is much larger than that of several earlier reported values (Table S1). However, the hydrogen evolution rate decreased with co-catalyst loading greater than 8 wt.%, as covering the 0D-2D CdS nanostructures surface with C@CoS2 nanostructures may inhibit photogenerated electrons from CdS under sunlight irradiation. This reduced the sunlight harvesting capability and decreased the H2 evolution rate of the prepared catalyst.39,40 The lower co-catalyst loading resulting in a decreased H2 evolution rate may be due to less active sites between the Co@CoS2 nanostructures and 0D-2D CdS nanostructures. Furthermore, the effect of the hole sacrificial agent concentration and photocatalyst dosage on the H2 evolution rate was evaluated, and the obtained results are shown in Figure S9 and S10. The results demonstrate that 1 mg of the catalyst and 20 vol% of lactic acid are ideal for efficient hydrogen evolution. In addition to the amount of H2 production, the photo stability is an important parameter for practical application of the catalysts. To understand the durability of the optimized CdS/C@CoS2 nanohybrid, further tests were performed to test its photo-stability. As shown in Figures 5(e) and 5(f), the nanohybrids are more stable and reusable for several hours and several


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recycling processes. These results show that the hydrogen production remained unchanged after five successive recycling rounds, but a slight decrease was observed at the third recycle (Figure 5(e)). This slight decrease is likely due to the oxidation of hole scavenger at catalytic reaction time. To verify this speculation, 3 mL of lactic acid solution was lent to the reaction system and the amount of H2 evolved rate was recovered in the fourth and fifth recycles rounds. This demonstrates that a continuous hydrogen production rate is mainly subjected on the accessibility of the hole scavenger in the catalytic system.41 The long-term stability results also demonstrate that the synthesized nanohybrid was stable for up to 50 h, as shown in Figure 5(f).



Figure 5. (a) Amount of H2 evolution under simulated sunlight irradiation with 0D CdS, 1D CdS, 2D CdS, 0D-1D CdS, 0D-2D CdS and 1D-2D CdS catalysts in lactic acid, 200 µL TFA, or a mixture of 20 vol% lactic acid and 200 µL TFA. (b) Hydrogen evolution rate of 0D-2D CdS catalysts with different amounts of TFA. (c) Amount of H2 evolved by using CdS/C@CoS2 nanostructures as the catalyst (d) Amount of H2 evolution in repeated experiments with optimized CdS/C@CoS2 nanohybrid. (e) H2 production rate in the repeated cycle experiment using the optimized CdS/C@CoS2 nanohybrid. (f) Long-term durability test of the optimized CdS/C@CoS2 nanocomposites. 22 ACS Paragon Plus Environment

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The excellent H2 production rate and stability of the synthesized 0D-2D CdS/C@CoS2 nanohybrid can be attributed to several factors. Based on the PL, DRS, photocurrent, and impedance analysis, the establishment of phase junction between cubic 0D CdS and 2D hexagonal CdS nanostructures likely served as an energy transition area to derogate the energy loss and transport photocharge carriers, reducing the recombination rate due to the favorable band structure.18,42 In addition, the water-soluble molecular co-catalyst serves as an accelerating hole transfer agent, which greatly reduces the recombination rate and enhances the photocatalytic hydrogen evolution rate.22 Also, the efficient light absorption capability of the 0D-2D CdS/C@CoS2 nanohybrid facilitates the light harvesting capacity and contributes to efficient photocharge-carrier generation, resulting in a high rate of H2 production. Finally, efficiently reduce the recombination rate by transporting the photogenerated electrons from the CdS phase junction to C@CoS2 nanostructures due to favorable band structures, as indicated by the DRS, PL, photocurrent, and electrochemical impedance analyses (Figure 4(a-d)).43 Based on the above experimental results herein, a plausible reaction path way for promoting H2 evolution under solar light is schematically presented in Scheme 2, with lactic acid as a hole sacrificial agent, TFA and C@CoS2 as co-catalysts, and 0D-2D heterophase CdS as a light absorber. The heterophase CdS nanostructure harvest the sunlight and produce the electrons and holes. The photogenerated electrons could follow one of the two pathways to reduce the recombination rate and improve the H+/H2 reduction capability. First, the excited photoelectrons could be transferred to the cubic phase CdS to hexagonal phase CdS nanostructures and effectively participate in hydrogen evolution. The second pathway features the shuttle the photo excited electrons from CdS to C@CoS2 nanostructures, which separates the photogenerated 23 ACS Paragon Plus Environment


charge carriers followed by efficient H+/H2 reduction. The molecular co-catalyst TFA accelerates the photogenerated hole transfer through a coupled reversible reaction TFA/TFA–, which excretes the main pathway of photogenerated electron–hole recombination and accelerates the oxidation of lactic acid, resulting significantly reduced recombination rate.22 Finally, we anticipate that the insights developed in this study may result in the utilization of low-cost CdS/C@CoS2 nanohybrids as an alternative for high-cost noble metal catalysts for photocatalytic H2 generation.

Scheme 2. Schematic reaction mechanism of photocatalytic H2 evolution by 0D-2D CdS/C@CoS2 nanocomposites as active catalysts under solar light irradiation with TFA as a molecular co-catalyst

CONCLUSIONS


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Highly robust low price photocatalytic nanohybrids for solar-driven H2 production by water splitting were designed and characterized herein. The designed photocatalytic system contains heterophase nano-junctions of 0D CdS mesoporous networks on porous singlecrystalline 2D CdS nanosheets as sunlight harvesting materials. Few-layered carbon-anchored CoS2 nanoparticles and water-soluble TFA were utilized as co-catalysts to hinder the photo charge carrier recombination generated from the CdS nanostructures. Benefiting from the huge active catalytic sites, high light harvesting capacity and suitable band structure, the nanohybrid exhibits a high amount of H2 87.73 mmol·gcat–1·h–1 was evolved. Moreover, the nanohybrid is stable for up to 50 h. We believe that the results presented here may lead to their use as noblemetal-free robust photocatalysts for the production of sustainable and green H2 fuel to meet future global energy demand.

ASSOCIATED CONTENT Supporting Information. Experimental details; High magnification FETEM image of CdS mesoporous nanostructures, X-Ray diffraction patterns of ZIF-67, Co@C and C@CoS2 nanostructures; Mott-Schottky plots of 0D-2D CdS and C@CoS2 nanostructures; Impedance spectra of CdS, and 0D-2D CdS/C@CoS2 nanostructures; Effect of different scavenger on H2 production rate of 0D-2D CdS nanostructures; Effect of photocatalyst loading on hydrogen production rate; Effect of concentration of sacrificial lactic acid on the hydrogen evolution rate; Comparison of photocatalytic H2 evolution rate reported in the literature using cobalt based nanostructures and their nanocomposites with CdS nanostructures with our present results. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION Corresponding Author *E-mail) [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by a 2-Year Research Grant of Pusan National University. REFERENCES (1)

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Enhanced Photocatalytic Hydrogen Evolution by Integrating Dual

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